“Primary elections have been held in 34 states, and now is the time for Republicans to unite behind Governor Romney and take our message of fiscal conservatism and job creation to all voters this fall,” Mr. Bush said in the statement.

The endorsement was hardly a surprise – Romney has also been endorsed by former President George H.W. Bush and by former first lady Barbara Bush. And it's less a ringing personal endorsement of Romney than it is a plea for party unity.

But it is still good timing for Romney, coming off a big win in Illinois that many see as a turning point in the race. And it adds to the sense of inevitability that Romney will be the eventual nominee.

While Romney has always come across as the choice of the GOP establishment – and has earned far more endorsements than any other candidates in the race – some of the biggest names in Republican politics have held out on giving their blessing.

His rivals sought to downplay the endorsement, with R.C. Hammond, a spokesman for Newt Gingrich, calling it "just the completion of the establishment trifecta" – a reference to Romney's previous endorsements from Bob Dole and Jeb Bush's father.

But expect more key endorsements in the days and weeks to come, and increasing pressure – both public and behind the scenes – for Romney's remaining rivals to drop out of the race.